Using Admixtures in Decorative Concrete Work
How-to tips from the pros that save time and money
by Cindy Grawl
of concrete mix design. They are those materials other than cement, water, and aggregate, and they are put into the mix
of concrete mix design. They are those materials other than cement, water, and aggregate, and they are put into the mix just before or during the mixture to help it meet the design and finish needs of the installation. Says Gabriel Ojeda, president of Fritz-Pak, Dallas, admixtures can be used in two ways: to improve concrete’s ultimate durability and strength and to modify its behavior to make it easier to work with.
They can reduce the cost of concrete work, modify the qualities of the hardened concrete, and assure the quality of the concrete during the mix, transport, pour and set, while they help to modify setting times in response to weather or workability needs and make the concrete stronger and more durable.
According to the Portland Cement Association, Skokie, Illinois, the effectiveness of an admixture depends on the type and amount of cement used, water content, mixing time, slump, and the temperatures of concrete and air: Admixtures include the following broad categories:
- air entrainment chemicals that trap air in the mix, allowing it to withstand freeze-thaw cycles by giving water a place to go;
- water reducers to increase the slump and enhance the concrete’s placeability without adding water and thus reducing the strength and permeability of the concrete; and
- accelerators and reducers to help modify set times and thus allow more time to do design work or keep the job on track with time constraints.
- other admixtures, says the PCA, can be used to color the concrete, inhibit corrosion, reduce shrinkage, enhance workability, damp proof and bond the concrete.
According to Master Builders, a manufacturer of admixtures based in Cleveland, Ohio, concrete containing these admixtures, meant to safeguard concrete in exterior installations from the rigors of Northern climates, is more workable. However, adding too much air entrainment can reduce the compressive strength of the concrete. The bubbles in air-entrained concrete, says Ojeda, make it stronger and easier to trowel, as well as reducing bleed to produce a better finish.
Bob Harris of L.M. Scofield, Los Angeles, agrees. “You can’t do the finish work in excessive bell water, so air entrainment lets you get to it sooner and assures better quality,” he says.
However, according to Kevin Copeland, manager of technical services for the Southwest Region at Boral Material Technologies, San Antonio, Texas because air entrainment slows the rate of bleeding, it can cause delamination if the surface seals before the bleed out, so beware.
Air-entrainment additives can be used to save contractors money if they have a load that will be used for both interior and exterior installation. They can do the interior pour first and then add the air entrainment chemicals that will help the concrete stand up to weather for the exterior pour.
These admixtures come in three strengths — normal range, mid range and high range, also known as superplasticizers — that improve workability and decrease the need for water in the mix by varying amounts, allowing for a stronger concrete without increasing the amount of cement, helping to save on costs. Adding water reducers, says Rick Seymour, technical services manager at Lafarge, Denver, Colorado, can help contractors get better detailing — he’s seen the engraving on a coin reproduced — and also save labor costs, as a four-man crew can do the work of six.
Lee Levig, of Levig Concrete is San Francisco, agrees that mid-range water reducers can give more “life” to the concrete — more detail time — while making concrete easier and less labor-intensive to place. They also make surfaces wetter, he says, so color hardeners can have a better effect.
The high-range water reducers, or superplasticizers, can produce highly flowable concrete while maintaining a specified water to concrete ratio, according to Master Builders, but their effect only lasts a short time. Fritz-Paks’ Ojeda adds that they do give the concrete more strength, by reducing the water to cement ratio, but that as it becomes denser, it may become harder to add color hardeners, as they need to absorb the water that just isn’t there. Another drawback, says Levig, is that too much of a plastic effect can result in shrinkage.
According to Boral’s Copeland, superplasticizers can be used to create self-compacting concrete, but that can cause aggregate to sink down and segregate out. To overcome this, he recommends use of viscosity modifying admixtures that will keep aggregates suspended throughout a mixture.
Superplasticizers can also lead to shrinkage if overused, and many contractors use fibers to assure that the concrete will stay put. However, warns Harris, polypropylene fibers are to be used only to overcome surface cracking; they will not give the bearing strength of rebar.
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